In lithographic printing systems, an original image to be reproduced is scanned by a scanner on a pixel-by-pixel basis, and the resulting scanned values are used to create one or more printing plates. When a monochrome (e.g., black and white) reproduction is to be printed, a single printing plate is produced. On the other hand, when a color reproduction is to be printed, a set of four plates are typically produced, one for each of the subtractive primary colors of magenta, cyan, and yellow, and one for black. The colored inks reproduce the hues of the original image, and the black ink produces a desired neutral density that cannot be attained by color inks alone. In addition, because black ink is less expensive than color inks, grey replacement (a form of undercolor removal) may be effected to replace quantities of the color inks with black ink. Such a process reduces the cost to produce the reproduction without significantly affecting the appearance thereof.
In traditional prior art lithographic half-tone reproduction systems, each printing plate includes a number of contiguous cells of equal size wherein each cell contains zero, one, or more elementary marks (or "microdots") clustered together to form a single large "dot" in the cell. More recently, systems have been devised wherein microdots are dispersed in a regular pattern in each cell. In other systems, microdots are dispersed in a random pattern on a medium. In each system, the number of elementary marks used to create a clustered dot, or a dispersed dot, depends upon the amount of ink to be applied to the substrate at the cell location. The amount of ink to be applied to the substrate at the cell location is, in turn, dependent upon the scanned value of the original image at a corresponding location thereof.
In the past, dots were formed within cells on a regular spacing or grid using a screen in a photochemical etching process. More recently, half-tone reproduction systems have utilized data processing equipment that electronically produces data representing a half-tone image. This data can be used to plot film or to directly form a printing plate without the use of an actual screen. However, the terms "screen" and "screening" are still used to define the dot pattern produced in a half-tone reproduction. For example, the term "screen ruling" specifies the distance between centers of adjacent cells of the plate. When the cells are all of the same size and regularly spaced, the plate is said to have a "regular screening." In such a case, a cell contains one period of the "screen."
Systems that reproduce half-tone images with regular screening have several drawbacks. For example, resolution is limited by screen ruling. Screen ruling is limited, in turn, by the minimum dot size and spacing that can be reliably and consistently printed. Moreover, regular dot patterns produced by regular screening in color reproduction result in moire effects and color shifts caused by interference between the superimposed dot patterns. Such undesirable artifacts have been reduced in the past by superimposing the screens at angles with respect to one another. However, this technique is not entirely satisfactory because undesired effects are only minimized, not eliminated completely.
The prior art has reduced the effects of moire and color shifts while at the same time enhancing the quality of the reproduction by eliminating the use of regular screens. Instead, a process known as "screenless" lithography (also referred to as random screening or random dot lithography) has been used to produce irregular dot patterns on the printed page. The use of irregular dot patterns can eliminate visual interference caused by superimposition of the dot patterns, and hence moire effects are substantially reduced or eliminated.
In one prior art system, a printing plate having an irregular grain structure is photographically exposed and chemically developed in a photolithographic process to produce an irregular dot structure. Such systems, however, cannot create consistent dot patterns from plate to plate.
Random screening has been electronically achieved using, for example, an error diffusion technique, such as the error diffusion technique described by Floyd and Steinberg in their paper "An Adaptive Algorithm for Spatial Greyscale," Proceeding of the S.I.D., Vol. 17/2, Second Quarter, 1976. This paper discloses a reproduction system that compares each continuous tone value, which is sometimes referred to as a contone and which is obtained by scanning an original image, with a threshold to obtain a binary approximation of the continuous tone value. When a continuous tone value is less than the threshold, the continuous tone value is converted to a binary zero. On the other hand, if the continuous tone value is greater than the threshold, the continuous tone value is converted to a binary one. After conversion, the error resulting from the approximation of the continuous tone value is subdivided into error portions, and the error portions are summed in a prescribed pattern with neighboring continuous tone values yet to be converted so that such error is diffused. Each continuous tone value to be converted is thus a combination of its original continuous tone value plus any error portions diffused to it by the conversion of neighboring, previously converted continuous tone values. This process is repeated for each continuous tone value until all continuous tone values resulting from scanning of the original image have been converted to binary values. The result of this conversion of the continuous tone values produced by scanning an original image is typically a bit map. The bit map thus derived is used to produce a reproduction of the original image where the reproduction has dots at locations defined by the binary values in the bit map.
While the foregoing process is effective to reproduce half-tone images with random dots, it has been found that the dots still create artifacts in the reproduction. These artifacts detract from the visual appearance of the reproduction. The conversion system disclosed in Xie, et al. U.S. Pat. No. 5,335,089 assigned to the assignee of the instant application further reduces visible artifacts and perception errors by using a filter in order to remove noise which arises in the conversion process. In this system, a first error is produced by applying the filter (i) to the binary values resulting from the previous conversion of a selected number of continuous tone values which neighbor the continuous tone value undergoing conversion, (ii) to the binary values which result from a predicted conversion of a certain number of continuous tone values which are yet to be converted and which neighbor the continuous tone value undergoing conversion, and (iii) to a binary value having an assumed value of zero for the continuous tone value undergoing conversion. A second error is produced by applying the filter (i) to the binary values resulting from the previous conversion of the selected number of continuous tone values which neighbor the continuous tone value undergoing conversion, (ii) to the binary values which result from the predicted conversion of the certain number of continuous tone values which are yet to be converted and which neighbor the continuous tone value undergoing conversion, and (iii) to a binary value having an assumed value of one for the continuous tone value undergoing conversion. The continuous tone value undergoing conversion is then set to a value of zero if the first error is less than the second error, and to a value of one if the first error is greater than the second error. Each continuous tone value is thus converted to a corresponding binary value which is typically stored in a bit map for rendering by a marking device.
Another electronic system for converting continuous tone values to binary values is described by Stoffel and Moreland in their paper "A Survey Of Electronic Techniques For Pictorial Image Reproduction," IEEE Transactions on Communications, Vol. Com-29, No. 12, December 1981. As described in this paper, a continuous tone value is converted to one of seventeen different dot patterns. The particular dot pattern to which the continuous tone value is converted depends upon the magnitude of the continuous tone value. Each of the seventeen dot patterns is confined in a four-by-four cell grid. These dot patterns include a first dot pattern having no marks in the cells, a second dot pattern having a mark in one predetermined cell, a third dot pattern having marks in two predetermined cells, a fourth dot pattern having marks in three predetermined cells, . . . and a seventeenth dot pattern having marks in all sixteen cells. Thus, the dot patterns to which continuous tone values may be converted have only seventeen possible densities. Each continuous tone value produced by scanning an original image is converted to a corresponding dot pattern, and each dot pattern is typically stored as a bit map for rendering by a marking device.
None of these continuous tone value to binary value conversion systems compensate for the tendency of real world marking engines to mark imperfect dots on substrates, such as printing plates, film, and/or paper. That is, the properties of a given marking device, and the properties of a substrate receiving marks from the marking device, can affect the optical density of a bit map rendered on the substrate. For example, the optical density measured on paper when a device has imaged 50 percent of the addressable dots (i.e., half of the dots were made black) may not be half of the optical density of the paper when the same device blackens every point on the paper. Similarly, making 40% of the dots black may yield a density other than 40% of the full black density, and so on.
Such properties, which result in an inequality between the density of the input continuous tone values and the density of the resulting rendered dot structure, are referred to herein as adjacency effects. As one result of adjacency effects, when two marks (i.e., pixels) are placed so that they are adjacent to one other, either horizontally, vertically, or diagonally, they often leave a mark on the media which appears to be larger or smaller than the combined area of the two separate marks. Consequently, two patches marked on a printing media, such that the patches have the same number of marks per unit area, can produce significantly different density measurements when measured with a densitometer. This type of behavior results in a reproduction which is less than a true rendition of an original.
The present invention addresses one or more of the problems described above.